Many different means of security are available to prevent duplication of printed commercial documents, such as water marked paper and fluorescent inks which form latent images. The use of latent images as a security measure is well known but has limited utility where original documents are altered or modified. Fraudulent manipulation of print on issued money orders, checks, transaction receipts and legal documents is a significant problem. Alteration and modification of conventional print is possible in that the conventional inks employed typically comprise pigments/dyes and binders which are soluble within aqueous and/or organic solvents. For example, in ink jet printing, impact printing and relief printing, the pigment or dye is disposed within a solvent carrier which is subsequently evaporated and/or absorbed in the paper to cure the ink. The solubility characteristics of the pigments, dyes and binder provide for weak or reversible bonds to the recording medium. In addition, many techniques require certain coatings on the fibrous web or base sheet of papers to provide high print density and print with high definition. Such coatings also enable modification of print on the paper in that they typically form a barrier between the ink and the fibrous web of the paper and comprise components which are soluble in aqueous or organic solvents. This enables removal of portions of the coating with print thereon or the print itself.
The use of reactive components in inks and paper to improve the quality (water-fastness and smear resistance) of prints from inks is known. Examples of such inks and papers are described in the following references.
U.S. Pat. No. 5,006,862, issued to Adamic et al., describes the use of reactive dyes which attach to a chromophore wherein the prints are treated with a strong base to improve water-fastness and smear-resistance.
U.S. Pat. No. 5,139,860, issued to Mori, describes papers containing urethane resins and epoxy resins usefull in forming thin papers for heat-sensitive stencil printing.
U.S. Pat. No. 4,694,302, issued to Hackleman et al., describes a method of increasing the water fastness and print quality of an ink employed in ink jet printers with the use of a reactive species that react with a cellulose component in the substrate or added thereto to form a polymer that binds the dye in the ink to the polymeric lattice.
There is still a need for a printing system which produces printed media which cannot be modified, either chemically or mechanically, without detection.